This disclosure relates to abrasion resistant coatings. More specifically, this disclosure relates to coatings that afford protection against abrasion, wear, vibration, solid particle erosion and liquid particle erosion.
Rain and sand erosion damage on leading edge surfaces of helicopter rotor blades, propeller blades, aircraft wings, radomes, and antenna are well-known problems. Metallic coatings derived from metal such as, for example, nickel, titanium, stainless steel, or the like, generally provide good resistance to erosion damage. These metallic coatings, however, have many disadvantages. They have poor sand erosion resistance and may spark upon impact with sand particles. In addition, they reflect light and radar signals, making them undesirable for military operations.
In order to overcome problems with the metallic coatings, coatings comprising polymers (polymeric coatings) have been used. The polymeric coatings are generally modified to have a low gloss by the incorporation of flatting agents. They are additionally modified by the incorporation of dyes and colorants to have a color similar to that of the rest of the aircraft, in order to minimize detection. These addition of the flatting agents and colorants to the coating reduces the ductility of the coatings and makes them more susceptible to rain and/or sand erosion. In particular, the use of flatting agents to attain low gloss causes a decrease in the elongation to break and a substantial reduction in the rain and/or sand erosion resistance.
In U.S. Pat. No. 4,110,317 to Moraveck, urethane coatings having improved weather and protective properties are provided by applying and curing in an atmosphere containing moisture a coating composition comprising (1) an isocyanate-terminated prepolymer comprising of the reaction product of (a) a polytetramethylene ether glycol having an average molecular weight between about 500 and about 700, (b) an oxyalkylated triol having an average molecular weight between about 400 and about 1000 in an amount between 0 and about 20 percent by weight based on the combined weight of the oxyalkylated triol and the polytetramethylene ether glycol, and (c) an organic diisocyanate and (2) an inert organic solvent. However, the percent elongation at break for these compositions is not very high.
In military specification, MIL-C-85322B (2), titled “Coating, elastomeric, polyurethane, rain erosion”, published in 1999 by the Naval Air System Command, the specification calls for polyurethane coatings based on TDI-prepolymer and a ketimine or other amine type curing agent. The disclosed physical property requirements are a minimum tensile strength of 1000 psi and a minimum elongation at break of 350% as determined by ASTM D2370. No mention of Shore A hardness or tensile set recovery are disclosed. Chemglaze M331 and Aeroglaze M1433, both manufactured by Lord Corporation, are qualified under MIL-C-85322. The coatings are based on TDI prepolymer and a ketimine (methylene dianiline). There is no disclosure of physical properties in the M1433 product datasheet. The M331 datasheet discloses that the physical properties of a cured M331 coating has a tensile strength of 350 kg/cm2, an elongation at break of 500% and a Shore A hardness of 95. Aeroglaze is available only in gloss gray color and Chemglaze only in gloss black color. No low gloss versions are available to meet the demand of the modern military requirements.
In military specification, MIL-C-83231, titled “Coatings, polyurethane, rain protective for exterior aircraft and missile plastic parts”, issued in 1969 and later replaced by SAE-AMS-C-83231 in 1999, the specifications call for polyurethane coatings based on moisture or non-moisture curing mechanism. There is no reference to the requirements of physical properties in tensile strength, elongation at break, tensile set at break (recovery) and Shore A hardness. CAAPCOAT B-274 and AS-P108 available from Caap Co. was approved under this specification.
CAAPCOAT B-274, available in black, is a polyurethane rain erosion coating qualified under MIL-C-83231 A, Type II, Class A, and Composition L. According to its Materials Safety Datasheet (MSDS), it is based on methylene bis(4-cyclohexylisocyante terminated polyester prepolymer. The curing agent is based on aliphatic amine. The accelerator is dibutyl tin dilaurate. CAAPCOAT AS-P108 antistatic polyurethane rain erosion coating is based on TDI-isocyanate terminated polytetramethylene glycol prepolymer. Its catalyst contains triethylene diamine and dipropylene glycol.
In MIL-C-83445, titled “Coating system, polyurethane, non-yellowing white, rain protective, thermally reflective”, issued in 1974 and later replaced by SAE-AMS-C-85445 in 1999, the specification calls for polyurethane coatings based on aliphatic or cycloaliphatic prepolymers. There is no reference to the requirements of physical properties in tensile strength, elongation at break, tensile set at break (recovery) and Shore A hardness. The only commercial product qualified under this specification was Caapcoat C-W4. Its product datasheet discloses that it has a minimum tensile strength of 210 kg/cm2 and a minimum elongation of at break of 350%. There is no disclosure of tensile set at break (recovery) and Shore A hardness. According to its MSDS, it is based on methylene bis(4-cyclohexylisocyante terminated polyester prepolymer. The curing agent is an aliphatic amine. The accelerator is dibutyl tin dilaurate.
The above three military standards do not specify the 85 degree gloss requirement, and hence the aforementioned coatings are supplied in high gloss black, gray or white. However, the present day military applications specify low gloss sprayable coatings with an 85 degree gloss values of about 3 to about 5.
Erosion resistance coatings can generally include a base coat layer and a top coat layer. A widely used commercial polyurethane protective coating system in low gloss gray is CAAPCOAT FP®-250, which consists of CAAPCOAT® FP-200, a high gloss polyurethane base coat and FP-050, which is a lusterless polyurethane top coat. CAAPCOAT FP®-200 color matched gloss polyurethane rain erosion coating is based on chemistry similar to the C-W4. Its technical data sheet indicates that it has a minimum tensile strength at break of 210 kg/cm2 and a minimum elongation at break of 350%. The FP-050, lusterless top coat, has a minimum tensile strength of 315 kg/cm2 and a minimum elongation at break of 300%.
None of the above sprayable coatings are satisfactory for today's military applications as low gloss erosion protection coatings and there is therefore a need for protective coatings that can meet present day military specifications.